The present invention relates to a thermally stable fire retardant laminate for printed circuit boards using a thermally stable aluminium hydroxide without the need for other fire retardant agents.
Printed circuit board laminate types are defined by the American NEMA (National Electrical Manufacturers Association) standard, the terminology is accepted world-wide. Generally, laminates are categorized according to the type of reinforcement used, i.e., cellulose paper or glass. Typically, the type FR-2 uses paper only, CEM-1 uses paper and woven glass, while type CEM-3 contains both woven and non-woven glass. The type FR-4 contains woven glass only.
To achieve the required V0 level of fire retardancy according to American Underwriters Laboratories"" standard UL-94, it is necessary to add either fire retardant chemicals to the polymer system or build halogens or phosphorous into the backbone of the polymer itself On combustion, these additives help to extinguish the fire. However, in the process of doing so they produce toxic and corrosive gases. In the case of combustion of FR-2 laminates, which contain phosphorous compounds, phosphoric acid is formed. CEM-1, CEM-3 and FR-4 laminates which contain brominated epoxy resin, produce corrosive and toxic hydrogen bromide on combustion.
It is well known in the art that aluminium hydroxide may be used to improve the fire retardancy of synthetic polymer systems based on for example epoxy, polyester, and vinyl ester because the polymers decompose in the same temperature range as the aluminium hydroxide However, the thermal stability of the gibbsite form of aluminium hydroxide (Al(OH)3 or sometimes represented as Al2O3.3H2O) is insufficient at the temperatures used to solder components to a printed circuit board laminate. This can result in blistering of the laminate thereby rendering it unusable.
It is known that when gibbsite type aluminium hydroxide is heat treated in air it is partially converted to the mono hydrate form, boehmite (AlOOH or Al2O3.H2O) which improves thermal stability but to the detriment of fire retardancy.
Japanese patent disclosure JP-A 60/203 438 describes a CEM-3 laminate containing heat treated gibbsite type aluminium hydroxide which has improved thermal stability over standard aluminium trihydroxide but which does not give the required level of fire retardancy so that brominated epoxy resin has to be used to achieve the UL 94 V0 rating. In such circumstances, and in the absence of superior fire properties, other inorganic materials such as talc or clay could also be used.
The object of the present invention is to produce a CEM-3 laminate which has good thermal stability, is halogen/phosphorous free and meets the UL 94 V0 requirement. A further object of the invention is to develop a thermally stable aluminium hydroxide which impart the required characteristics to the laminate.
It was possible to achieve the object of the invention by means of a laminate for printed circuit boards according to the invention, by a method for the preparation of the laminate according to the invention, by a printed circuit board made of the laminate according to the invention, and by a thermally stable aluminum hydroxide according to the invention.
The CEM-3 type laminates are as a rule constructed from two layers of woven fiberglass on the outsides and three layers of non-woven glass tissue in the core. According to the invention, the laminate for printed circuit boards has surface layers comprising curable resin-impregnated woven glass fabric and intermediate layers comprising curable resin-impregnated non-woven glass, and is characterized in that the intermediate layers contain 200% by weight to 275% by weight (based on the resin) of a thermally stable aluminum hydroxide of the molecular formula Al2O3xc2x7nH2O wherein 2.6 less than n less than 2.9.
The curable resin may be an unsaturated polyester resin, an epoxy, a vinyl ester or any suitable thermosetting compound which on combustion decomposes in the same temperature range as the thermally stable aluminium hydroxide.
The laminates may be based on epoxy resin which allows a batch process to be used. The laminates may also be produced by continuous process using unsaturated polyester or vinyl ester, i.e., resins which polymerize via a free radical mechanism. The nature of the invention however does not limit the manufacturing technique for the laminate.
When the production of the laminate is based on epoxy resin, as a rule two layers of woven glass preimpregnated with the epoxy resin are combined with three layers of non-woven glass preimpregnated with epoxy resin containing thermally stable aluminium hydroxide These five layers are usually then combined with one or two layers of copper foil on the outside and the assembly subjected to heat and pressure to polymerize the resin and consolidate the laminate.
Most epoxy resins used for electrical and electronic applications are derived from bisphenol A or cycloaliphatic species. The most common curing agent is dicyandiamide.
According to the method of the present invention, the intermediate layer comprising curable resin impregnated non-woven glass contain 200% by weight to 275% by weight, preferably 225% by weight to 250% by weight, of the thermally stable aluminium hydroxide relative to the weight pf the curable resin.
In the molecular formula, Al2O3.nH2O, n preferably has a value of 2.7 to 2.8.
A feature of the halogen and phosphorous free epoxy laminate of the present invention is the use of thermally stable aluminium hydroxide as the sole fire retardant.
The prior art describes the removal of a part of the water content of aluminium hydroxide as a means of improving it""s thermal stability. If insufficient water is removed, the laminate containing the aluminium hydroxide does not survive the soldering operation. However, a limitation of the prior art is that if too much water is removed the resulting aluminium hydroxide may contain too little water of constitution to act effectively as a fire retardant.
A further complication is the formation of the more thermally stable phase aluminium hydroxide (boehmite), which not only contains one third of the amount of water as the original aluminium trihydroxide (gibbsite) it also holds it back to 520xc2x0 C. before releasing it. This detracts further from fire retardant effectiveness.
It is well known in the art that the finer is the aluminium hydroxide starting material, the less is the tendency to form the undesirable boehmite phase on heating. However, at the particle sizes necessary to avoid boehmite formation ( less than 1 xcexcm), it becomes virtually impossible to process the material in synthetic resin. On the other hand, thermally stable aluminium hydroxide which is processable will contain too much boehmite thus preventing the production of a halogen-free epoxy laminate which passes UL 94 V0.
Essentially, the novel feature of the thermally stable aluminium hydroxide of the present invention is the surprisingly low boehmite content relative to the average particle size. This allows lower values of n to be obtained thereby improving water stability without compromising on fire properties"" effectiveness. Thus, for n less than 2.6, there is insufficient water (and too much boehmite) to achieve the required fire properties whereas for n greater than 2.9, there is too much water still present to achieve the required thermal resistance to soldering.
The decoupling of the relationship between boehmite formation and particle size is achieved by suitable mechanical treatment of relatively coarse particles (average size ca. 60 xcexcm) of aluminium hydroxide prior to the thermal pretreatment step. A suitable mechanical treatment would be provided for example by the application of compressive forces to aluminium hydroxide to increase polycrystallinity. Without being bound to any particular theory, it appears that structural degradation by such means creates the conditions for a more easy diffusion of water to the exterior of the crystals thereby minimizing the tendency to build-up of hydrothermal pressure within the crystals and hence the reduced tendency to boehmite formation.
In practical terms, the product of the invention can be created by a size reduction of an aluminium hydroxide agglomerate with an average particle size D50% of 40 to 80 xcexcm, preferably of 50 to 70 xcexcm, crystallized out from a typical Bayer processxe2x80x94sodium aluminate solution.
Any size reduction technique, e.g. ball milling, which separates the crystals present within coarser agglomerates with little or no gross fracture of the single crystals can be applied. A simultaneous broadening of the particle size distribution due to attrition and the absence of crystal breakage by gross fracture improves the processability of the thermally stable aluminium hydroxide in the respective resin system.
The subsequent thermal stabilization can be achieved by simply heating the material recovered from the size reduction treatment at a temperature and during a time sufficient to reduce the water loss on ignition from 34.5 wt % (n=3) to a level corresponding to the n value according to the invention.
The thermally stable aluminium hydroxide according to the present invention expediently has a specific surface area measured according to the BET method of from 2 to 10 m2/g. A very essential point is the particle size distribution which combines a relatively fine average size with a breadth of particle sizes. This improves the dispersion of the thermally stable aluminium hydroxide in the resin while at the same time minimizing the tendency of the coarser particles to sediment out during processing and avoiding any filtering effects by the non-woven glass an optimum particle size distribution allow minimum viscosity of the resin/filler mix at a filling level sufficiently high to achieve the desired flame retardant properties of the laminate without the need to add any other flame retardants.
The average particle size of the thermally stable aluminium hydroxide of the present invention D50% is in the range 5 to 10 xcexcm. The breadth of the particle size distribution is indicated by the D10% range, i.e., 10% by weight of the particles are smaller than 0.5 to 1.5 xcexcm and the D90%, range, i.e., 90% by weight of the particles are less than 20 to 35 xcexcm in size.
The oil absorption measured according to standard method DIN 53199 is within the range 25 to 35 ml/100 g.
The incorporation of the thermally stable aluminium hydroxide into the curable resin can be accomplished by methods known to those skilled in the art, i.e., usually by introducing the filler into the predissolved mixture of resin and curing agent using suitable equipment such as shearing head mixer. If needed, other inorganic thermally stable fillers may be added to the formulation such as fine silica, clay or talc, although none of these would significantly enhance fire properties.
Further processing of the resin/filler mix to the xe2x80x9cprepregxe2x80x9d stage and then to the cured laminate is common state of the art and described in the literature, an example of which is xe2x80x9cHandbook of Epoxide Resinsxe2x80x9d, published by the McGraw-Hill Book Company.
The cured laminate according to the present invention shows excellent thermal stability, when immersed in molten solder at 260xc2x0 C., it shows no signs of blistering or bubbling for a period of over 90 seconds The laminate also has excellent fire retardancy characteristics and meets the requirements of UL 94 V-0.